Patent GB-813039-A (GB)
Title: Improved ultrasonic thickness gauge
IPC: G01B17/02
Filed: 1956-03-20
Abstract: 813,039. Ultrasonic inspection devices. SPERRY PRODUCTS Inc. March 20, 1956 [March 21, 1955], No. 8738/56. Class 118 (2). Relates to an ultrasonic system for measuring the thickness of a specimen 10 in which each transmitted pulse applied to the front surface of the specimen produces a plurality of echo pulses by multiple reflection between the front surface and the rear surface, a sawtooth wave G, Fig. 2, being initiated by the transmitted pulse and terminated after reception of a predetermined number of echo pulses so that the amplitude of the sawtooth is proportional to the thickness. According to the present invention a sharp transmitted wave pulse is produced by applying from a generator 12 a wave pulse A (terminated by a damping circuit 21) to a driving piezoelectric crystal 17 which is coupled to a driven piezo-electric crystal 18 in contact with the specimen 10 by means of an element 19 of effective length greater than the effective thickness of the specimen 10, the natural frequency of the crystal 17 being equal to the wave frequency of the pulse and the natural frequency of the crystal 18 being much higher than the wave frequency so that the crystal 18 is critically damped; crystals 17 and 18 are electrostatically shielded from each other. The electrical output D from crystal 18 comprising the direct pulse occurring at time T2 and the multiple echo pulses occurring at times T4 and T5 are applied through an amplifier 22 to a pulse counter 23 both of which are gated on by a pulse C initiated at time TI by the trailing edge of a pulse E from a generator 24 which is triggered by a pulse B from the generator 12. The direct pulse output from amplifier 22 triggers at time T2 a pulse generator 26 whose output F gates on the generator 27 producing the sawtooth G and the pulse counter 23 produces an output pulse after receipt of a predetermined number, e.g. one or two as shown respectively by the full-line and dotted portions of Fig. 2 C, F, G and H, of echo pulses. This output pulse is applied (1) to generator 25 to terminate the gating pulse C, and (2) to generator 26 to terminate the pulse F controlling the sawtooth G. The sawtooth G is applied to a peak rectifier 38 which is discharged by a pulse B from generator 12, the output H from rectifier 38 being filtered at 29 to give a D.C. voltage T of amplitude proportional to,the amplitude of the sawtooth G which is applied to a meter 30 and to control the frequency of a tone generator 31 whose output K is applied to an aural indicator. The counter 23 is reset by a trigger pulse B from generator 12. If no echo pulses are received a discriminator 32 operates to terminate the sawtooth G and operate a warning lamp 33 when the amplitude of the sawtooth G exceeds the value corresponding to full scale deflection of the indicator. The scale of the indication is controlled by adjusting the slope of the sawtooth G by means of control 27<SP>1</SP>. The slope of sawtooth G may be logarithmic to give a non- linear indication. When it is required to indicate only that portion of the magnitude of the thickness that exceeds a predetermined value, the initiation of the pulse F which gates on the sawtooth G is delayed by an amount corresponding to said predetermined value by applying the direct pulse from amplifier 22 to the generator 26 through a delay multivibrator 28. Specification 813,040 is referred to. Reference has been directed by the Comptroller to Specification 779,690.